This dimension really depends on the equipment you use (or plan to eventually use), and also how you use your equipment. Consider the table depth difference required to comfortably reach a FT-818 or IC-7300, vs say an ICOM-7851 or FT-9000, or large vintage equipment or power amplifiers.
Also if you're the type that mostly parks your equipment on one or a couple frequencies, reach distance isn't nearly as important compared to a situation where you need to reach your equipment often. Personally I suggest 30 to 36 inch table depth. With this depth, if your reach is too far you can always move your equipment further forward, but if the table depth is not deep enough, that can make for little or no arm/elbow support and uncomfortable operating.